The present invention relates to a comparator for providing an output signal having a hysteresis characteristic with respect to an input signal, and more particularly to a hysteresis circuit composed of complementary insulated gate field effect transistors.
The heretofore known comparator having a hysteresis characteristic comprises an operational amplifier and a resistance voltage-divider circuit. The operational amplifier has a first input terminal supplied with an input signal voltage and a second input terminal. The resistance voltage-divider circuit has a first resistor connected between an output terminal and the second input terminal of the operational amplifier and a series connection of a second resistor and a reference voltage source connected between the second input terminal and a ground terminal. However, this known comparator has shortcomings because a hysteresis voltage may be varied due to the internal impedance of the reference voltage source and because the resistance ratio becomes too large to be realized in an integrated circuit. This ratio is between the first and second resistors which are used for obtaining a large width of the hysteresis characteristic.
An hysteresis circuit overcoming the above-mentioned shortcomings, has been described in U.S. Pat. No. 4,069,431 granted for N. Kucharewski and issued on Jan. 17, 1978. This circuit makes use of a current mirror amplifier (CMA) having a current gain which is controlled responsive to an output condition. More particularly, the CMA is connected to a differential amplifier, as an active load. The differential amplifier receives an input signal and a reference voltage. A switching transistor having a gate connected to the circuit output terminal is connected in parallel with a conduction path on the output terminal side of the CMA. If this switching transistor is an N-channel enhancement type insulated gate field effect transistor, when the amplifier circuit output terminal is at a low level, the switching transistor is non-conductive. Accordingly, the current gain of the CMA is not varied. Whereas, when the circuit output terminal is at a high level, the switching transistor becomes conductive, to lower the impedance of the conduction path of the CMA on the output terminal side. In other words, the gain of the CMA is varied. By using this gain variation, a hysteresis characteristic is provided.
However, this amplifier circuit has a shortcoming since a slight noise superposed on the input signal may change the output level, upon the output inversion. This is because the CMA gain would not be varied unless the voltage level of the circuit output terminal reaches one stable condition level after having moved from the other stable condition level. This shortcoming will be described below in more detail, assuming that the switching transistor is of an N-channel type.
When the input signal voltage is decreased and then reaches one input level at which the output condition is inverted, the output voltage is increased, for example, from a low level toward a high level. This increased output voltage exceeds a threshold level of the switching transistor, and then this transistor is turned on. Consequently, the gain of the CMA is varied. This gain variation is fed back to enhance the output condition variation, and hence the output level changes abruptly to the high level. Since the threshold level of the switching transistor is relatively low, the gain variation is generated by the slight level increase of the output voltage.
When the input signal voltage is increased and then reaches the other input level at which the output condition changes from a high level to a low level, the output voltage is decreased. However, as long as the decreased output voltage is higher than the threshold level of the switching transistor, it does not turn off. In other words, the gain variation of the CMA does not occur. As the input signal voltage further increases, the output level further decreases and then, when the output voltage becomes smaller than the threshold level of the switching transistor, it is turned off. Then, the gain of the CMA is varied. As a result of this variation, the output level is abruptly decreased to the low level. Thus, the gain variation of the CMA is not produced by the output voltage from the high level to the threshold level of the switching transistor. Since the threshold voltage is very low, it can be said that, at the inversion point of the output voltage from the high level to the low level, the gain variation of the CMA would not occur. Consequently, as the input signal voltage is increased, the output signal voltage is decreased gradually. For this reason, the noise superposed on the input signal might possibly cause a malfunction of a load circuit connected to the output terminal.
In order to eliminate this shortcoming, the above-identified U.S. patent provides a buffer amplifier such as an inverter which is connected between the differential amplifier and an output terminal. For instance, the inverter has its threshold level equal to one-half of the power supply voltage. When the input signal level exceeds the threshold level or becomes lower than the threshold level, the output condition of the inverter is changed rapidly. Therefore, even if the voltage at the output of the differential amplifier circuit terminal is descreased gradually as described above, unless it becomes lower than the threshold voltage of the inverter, the output voltage level of the inverter is not be inverted. When the output terminal voltage has become lower than the threshold level of the inverter, the output level of the inverter is changed. Accordingly, a steep output voltage waveform can be obtained by the added inverter.
However, it is to be noted that when the output condition of the inverter is varied, the gain of the CMA has not yet been varied. This means that a noise superposed on the input signal is also input to the inverter. For this reason, the input voltage level of the inverter is varied by the noise. In other words, the output condition of the inverter becomes indeterminate due to the noise. As described above, the amplifier circuit having a hysteresis characteristic, according to the prior art, lacks in the stability of the output condition with respect to noises.
Moreover, according to the prior art, there was another shortcoming that one of the input signal voltages at which the output condition is inverted is fixed at the reference voltage.